This invention relates to four-wheel steering systems for automotive vehicles. More specifically, this invention relates to improvements in the determination of the rear wheel steering angle based on vehicle velocity, the magnitude of the front wheel angle and the rate of change of the front wheel angle.
Front wheel steering has been used in automotive vehicles since their inception one hundred years ago. The steering system is a major system required for driver operation of the car or truck and control of the path of the vehicle over the ground. The driver uses a handwheel that turns a steering column. The column is typically connected through rack and pinion gear and additional mechanical linkage to the front wheels. The angle between the front wheel plane and the longitudinal axis of the vehicle is the front wheel angle or steering angle. This wheel angle is proportional to the rotational angle of the driver""s handwheel. There is resistance to the turning of the steerable wheels in a vehicle and in the modern vehicle the driver""s front wheel steering effort is usually assisted by a power steering system.
Some vehicles, especially pickup trucks, may be designed with a rear wheel steering system that compliments the front wheel steering system. Pickup trucks have a relatively long wheel base and wide track. They are used in situations requiring low speed maneuverability and high speed steering stability. It is possible to improve both requirements with four wheel steering. Since the vehicle operator cannot independently operate two steering systems, obviously, the front and rear wheel steering systems must be coordinated. While at one time in the development of automotive vehicles there may have been mechanical systems that permitted four-wheel steering, the modern vehicle utilizes electronic control systems.
In a typical rear wheel steering system, an actuator independent of the driver action is provided to steer the rear wheels. For example, an electric motor is used as prime mover to drive a tie rod connected to the control arm of the steerable rear wheels, similar to the design of a typical electric power steering for front wheel steering systems. The control of the rear wheel steering is based on information from a position sensor on the steering column continually reporting the driver handwheel angle, and sensors on the wheels of the vehicle for continual determination of vehicle speed (i.e., velocity, in kilometers per hour, kph). An electronic controller is used to continually control the operation of a rear wheel steering actuator based on present inputs of the front wheel steering angle and vehicle speed. The controller reads the operator""s handwheel position sensor, and determines the amount of vehicle front wheel steer by dividing the handwheel position with the fixed steering gear reduction ratio that typically ranges from about 14 to 18.
When the front wheel angle is zero, the rear wheel angle will also be zero, usually regardless of vehicle speed. But when the front wheel is being turned at a particular vehicle speed, usually the rear wheels are also to be turned in a manner that supports the drivers turning effort. The issue is how to convert the driver""s front wheel steering and vehicle velocity commands to a suitable rear wheel angle steering command.
At the present state of the art, the practice is to predetermine suitable rear wheel to front wheel steering ratios, R/F, over the range of operating velocities for the vehicle. These R/F vs. velocity values, or processes for calculating them, are stored in the electronic database of the controller. During each steering control cycle, the controller looks up or calculates the appropriate R/F ratio for current vehicle velocity and multiplies it by the present front wheel angle to determine the new rear wheel angle command.
The R/F value is sometimes called the rear wheel angle to front wheel angle xe2x80x9cgain.xe2x80x9d The present methods of establishing gain values are based on the physical characteristics of the vehicle. And the gain values are set so as to achieve predetermined goals in vehicle handling performance. At relatively low vehicle speeds the rear wheel angle is set opposite to the front wheel angle to permit a smaller turning radius and at higher vehicle speeds the rear wheel angle is in the same direction as the front wheel angle for vehicle stability. The gain values are typically determined for steady state front wheel angles and vehicle velocities. They are based on the maximum rear wheel angle for current speed and front wheel angle to achieve minimum side-slip motion, or skidding, under most driving conditions. Gain values can be determined experimentally or using computer models of vehicle dynamics based on the design characteristics of the vehicle. Relevant vehicle physical characteristics include, for example, mass (sprung and unsprung), location of center of gravity, wheel-base, track (i.e., the lateral distance between wheels), and yaw constants.
FIG. 2 shows typical curves of the R/F ratio vs. vehicle velocity for a half-ton capacity pick-up truck, with and without the use of a trailer. It is seen in FIG. 2 that a portion of each curve displays negative gain, and a portion displays positive gain values. Each R/F ratio curve has one point crossing the axis of the vehicle speed, resulting in a zero gain at a certain value of vehicle speed. Only at that particular vehicle speed will the vehicle equipped with rear-wheel steering system behave like a regular two-wheel steer (2WS) vehicle. When the vehicle is further equipped with a trailer mode selection switch, the controller can determine which curve to use based on the input from such switch.
The state-of-the-art control of the rear wheel steering angle has the objectives to enhance the vehicle maneuverability at low speed and vehicle stability at high speed. These objectives can be achieved by a control methodology based on the physical characteristics of the vehicle. The practice is to steer the rear wheel to an opposite direction of the front wheel at low speed (so-called xe2x80x9cout-of-phasexe2x80x9d steering), and steering the rear wheel to the same direction of the front wheel at high speed (so-called xe2x80x9cin-phase steeringxe2x80x9d). Thus, at the present state of development, the amount of rear wheel steering is strictly dependent on the vehicle speed and the front wheel steer angle. The latter values are continually being sensed and the former value continually determined in open-loop calculation mode by the rear steering controller using predetermined R/F gain values.
Although it has been recognized that the rear wheel steering system under this state-of-the-art control can achieve the desired goals of vehicle maneuverability and stability, there are other issues related to driver""s feel that need to be addressed while attempting to achieve those goals. These issues include the on-center steering feel, the driver-induced lateral motion during transient maneuvers, and driver""s feel during usual straight-ahead driving at normal speeds. It is to be remembered that the only driving experience of most drivers is with front wheel steered vehicles. If the advantages of four wheel steering are to be realized on large pickup trucks and the like, operators must recognize and be comfortable with the handling of the vehicle as they try to steer it. It is a purpose of this invention to improve the state-of-the-art rear wheel steering control methodology for an enhanced driver""s feel under various conditions as well as for a certain degree of vehicle stability enhancement described below.
This invention provides three related improvements of rear-wheel steering control. The improvements are adjustments that are made to the basic R/F gain table used by the rear wheel steering controller in determining the rear wheel steering angle in response to input data of present front wheel angle and vehicle velocity. The first improvement is a deadband function which relates to minimizing rear wheel steering at low front wheel steering angles regardless of vehicle speed. The second improvement is a steering motion multiplier which modifies the rear wheel steering angle depending upon how fast the operator is turning the handwheel. The third improvement is a nonlinear gain multiplier which relates to adjusting the new rear wheel steering angle command depending upon the magnitude of the present front wheel angle.
The first improvement is based on the fact that much of normal driving is in a generally straight-ahead direction and only small driver handwheel corrections are required. The driver may be continually making small front wheel steering corrections. But in these circumstances no rear wheel steering assistance may be required even though the predetermined R/F gain values in the steering controller may provide for it. Accordingly, the first improvement comprises identifying such low front wheel steering angles before any rear wheel steering command is issued. The practice is termed imposing a xe2x80x9cdeadband functionxe2x80x9d to the process of using the operator handwheel position signal to initiate a rear wheel steering command. This deadband function is applied to the handwheel position signal before the signal is used for determining the amount of rear-wheel steering.
The purpose of this deadband function process is to preserve the vehicle on-center steering feel for the operator. The on-center steering feel relates to driver""s correlation between the handwheel torque and vehicle lateral acceleration when a small amount of steering action is taking place while driving straight ahead. The term xe2x80x9con-centerxe2x80x9d refers to the fact that the operator""s handwheel is generally xe2x80x9ccentered,xe2x80x9d meaning positioned at, or cycling closely around, zero degree. The consistency of such feel is one of the factors determining the quality of a steering system, and great amount of engineering effort is used to make sure a base vehicle has as good an on-center feel as it could do within the constraint of engineering and manufacturing cost. However, when rear-wheel steering is activated, it changes the amount of vehicle lateral acceleration given the same handwheel steering torque. As the R/F ratio varies over the speed range of vehicle operation, the amount of lateral acceleration caused by rear steering also varies. This can result in inconsistent on-center feel. Although it is absolutely unrelated to vehicle safety, the familiar feel of steering system, based on front wheel steering, is compromised.
The deadband function of this invention improves the vehicle on-center feel by not responding to the handwheel steering input when it is within a small range where on-center feel is critical. Such range is typically small, for example, five to ten degrees rotation of the handwheel position from zero front wheel steering angle. This, of course, amounts to a much smaller front wheel angle based on a typical 1/16 ratio between hand wheel turning angle and the resulting front wheel angle. The breadth of the deadband range can also be made as a function of vehicle speed. With such deadband function, the on-center steering feeling is better preserved without compromise of the rear-wheel steering for the purpose of vehicle low-speed maneuverability and high-speed stability.
The second improvement is the incorporation of a steering motion multiplier to the R/F ratio of rear-wheel steering control gain. In accordance with this aspect of the invention, once the R/F ratio is identified for current vehicle velocity from the controller look-up table, it is further multiplied by a suitable steering motion coefficient. To obtain the steering motion multiplier, vehicle steering motion must be first detected, and its degree of motion determined. This can be implemented in the rear wheel steering controller by processing the handwheel position information. One way of determining the steering motion is to detect the rate of motion of the handwheel by taking its time derivative. This value of the rate of change of the driver produced front steering angle is used to adjust the newly determined rear wheel angle.
The purpose of this improvement is to further enhance driver handling perceptions and vehicle stability beyond the benefits inherent in rear wheel steering using only the predetermined basic, steady state operation, R/F gain values. To maximize its effect, this gain may be further implemented as a function of vehicle speed. A speed-dependent implementation of this steering motion multiplier gain is illustrated in FIG. 8B. At a relatively low rate of steering motion the steering motion gain value is one and no further modification is made to the R/F gain. However, at low vehicle speed, 16 kph, and increasing handwheel turning rate, the steering motion multiplier becomes smaller and, accordingly, reduces the rear wheel steering angle under these steering conditions. Likewise, while at a higher vehicle speed, 120 kph, the steering motion multiplier also decreases but at a lower rate.
The third improvement is the incorporation of a nonlinear gain multiplier based on the current front wheel steering angle. Thus, with this improvement, the basic R/F ratio is further multiplied by a gain which is a function of handwheel angle position. This nonlinear gain multiplier has a relatively smaller value (i.e., less than 1.0) at a smaller handwheel angle, and reaches the value of 1.0 after a threshold handangle determined by vehicle performance requirement. FIG. 8A illustrates a typical implementation of this nonlinear gain multiplier. The purpose of this nonlinear gain multiplier is to maintain vehicle stability in high speed while maintaining a similar driving feel a driver is used to driving a regular 2-wheel-steer vehicle under the similar driving situation.
The incorporation of these process improvements in the operation of the rear wheel steering angle controller markedly improves the performance of driver and vehicle in actual driving situations. During driving with small front wheel steering corrections, rear wheel steering is minimized to preserve the driver""s on-center steering feel. And during times of sudden and/or substantial front wheel steering, the contribution of rear wheel steering is coordinated to help keep the driver in his/her two wheel steering feel comfort zone. The result is that the advantages of four wheel steering are preserved and better utilized by the driver in maneuvering large vehicles like pickup trucks with or without trailers.
Other objects and advantages of the invention will become more apparent from a detailed description of preferred embodiments that follows.